To deal rationally with the complexity of present communication systems and with the need to make different systems mutually compatible, the International Standards Organization ("ISO") developed a model for specifying such systems. Using this model, called the Open Systems Interconnect ("OSI") model, a communication system can be broken down into a hierarchial structure that permits standards to be defined at each level in the structure. The OSI model provides a hierarchy of seven different layers that can occur in a communication system. Each layer in the OSI model specifies a different function performed by the communication system.
The lowest layer in the OSI model, called the physical layer, specifies the physical structure of interfaces in a particular communication system or network. Thus, a standard for the physical layer of a communication system specifies such things as the number of wires, their electrical characteristics, the characteristics of signals transmitted over the wires, connectors used for joining two sets of wires into a single longer set of wires, etc.
The next higher layer in the OSI model, called the data link layer, specifies how data is transmitted error free through the communication system. Thus, a standard for the second layer in the OSI model specifies how to detect errors in transmissions passing over the physical layer, and how to correct any errors that may occur during transmission.
The next higher layer in the OSI model, called the network layer, specifies the manner in which connections are formed between various places in the communication system for transmitting data between them. The standard for the third layer in the OSI model, therefore, specifies the signals transmitted over the data link layer that cause the communication system to transfer data between two places on the network.
A recommendation by an International Telegraph and Telephone Consultative Committee ("CCITT") for the ISDN communication channel specifies the three lowest levels in the OSI model. Under the CCITT recommendation, a basic ISDN access consists of two full-duplex 64 kilobits per second ("kbps") digital data channels, called channel B1 and channel B2, plus another full-duplex 16-kbps digital channel, called a D-channel. Under the CCITT recommendation, using time division multiplexing, all three of these digital data channels may be transmitted over a single pair of twisted wires, or over two pairs of twisted wires. ISDN basic rate access, as specified by CCITT, was originally intended to provide a basic digital data transmission capability suitable for use by individuals such as in their homes or small businesses.
When ISDN basic rate access was initially specified, each of the B-channels was intended to carry either:
1. digital data, such as that from a personal computer or from a computer terminal; PA1 2. Pulse Code Modulation ("PCM") encoded digital voice communication; or PA1 3. a mixture of lower data rate communications including digital data and digitized voice that were each encoded at a fraction of each B-channel's full 64-kbps capacity.
Under the ISDN recommendation, the D-channel serves two purposes. First, the D-channel carries signaling information that controls the transmission of data over the two B-channels. In addition, when the D-channel is not carrying signaling information, it may be used to transmit packet-switching or low-speed telemetry. The combined data rate at which digital data may be transmitted over twisted pairs of wires in accordance with the ISDN recommendation for basic rate access is 144-kbps, i.e. 128-kbps for the combined B1 and B2 channels plus 16-kbps for the D-channel.
In addition to the ISDN basic rate access specified by CCITT, that organization has also specified a higher performance ISDN communication channel called primary rate access. Depending upon the particular geographic region of the world, using time division multiplexing an ISDN primary rate access has either twenty-four (24) time-slots or thirty-two (32) time-slots, each one of which carries the information of a single ISDN B-channel. The ISDN primary rate access having 24 time-slots is called a T1-connection. The ISDN primary rate access having 32 time-slots is called a El-connection. One way in which ISDN primary rate access uses these 24 or 32 time-slots is for each of 23 or 30 time-slots to carry independent B-channels of information with a remaining time-slot carrying a D-channel of control data.
Moreover, groups of the 23 or 30 time-slots in an ISDN primary rate access may also be used collectively to carry digital information at higher data rates. For example, using a connection identified as H0, six (6) time-slots of an-ISDN primary rate access may be dedicated to simultaneously carrying 6 B-channels of information. Thus, an ISDN H0-connection carries 384-kbps of digital information. In addition to the H0-connection, CCITT has also specified a service called a H11-connection which similarly consists of 24 B-channels. A H11-Connection may be established over an El-connection or alternatively over a T1-connection plus an independent D-channel of control data. Thus, a H11-connection simultaneously carries 24 B-channels of information. CCITT has also specified a service called a H12-connection in which an El-connection's 32 time-slots simultaneously carry 30 B-channels of information plus a D-channel of control data.
Building upon CCITT's ISDN recommendation, it has also established a recommendation, H.221, which specifies data structures used in transmitting audiovisual teleservices over channels having bandwidths from 56 to 1,920 kbps. CCITT's H.221 Recommendation specifies data structures for communicating audiovisual information over from 1 to 6 B-channels, from 1 to 5 H0-connections, or over either a H11 or a H12-connection. Using 1 to 6 B-channels for transmitting audiovisual information provides data transmission rates of 64 to 384 kbps in increments of 64 kbps. Using 1 to 5 H0-connections provides data transmission rates of 384 to 1,920 kbps in increments of 384 kbps. A H11-connection provides a data transmission rate of 1,536 kbps, while a H12-connection provides a data transmission rate of 1,920 kbps.
In addition to the H.221 Recommendation, CCITT has also established a recommendation, H.242, which specifies a protocol for establishing audiovisual teleservices. The H.242 Recommendation establishes procedures by which two audiovisual terminals intercommunicate prior to and during an exchange of audiovisual information to match their respective capabilities for transmitting and receiving audiovisual information. Included among the capabilities intercommunicated between two such terminals are their respective audio capabilities, video capabilities, transfer rate capabilities data capabilities, encryption capabilities, and bit-rate allocation signal ("BAS") capabilities.
FIG. 1 illustrates the relationship that exists among B-channels 22 and D-channels 24 of three ISDN basic rate accesses 26 and a layer 28 corresponding to the CCITT H.221 Recommendation. The B-channels 22 and the D-channels 24 belong to a layer 30 that implements the CCITT I.400 Recommendation. In the illustration of FIG. 1, an arrow 32 indicates the passage of time from the beginning of an audiovisual telecommunication until its end. FIG. 1 also illustrates the superposition over each D-channel 24 of a layer 34 that implements a CCITT Q.921 Recommendation, and a layer 36 that implements a CCITT Q.931 Recommendation. The layers 34 and 36 provide independent call set-up and tear-down for each of the ISDN basic rate accesses 26. Sets-of double-headed arrows 38 extending between each of the B-channels 22 and the H.221 layer 28 indicate bidirectional exchanges of audiovisual information between the two layers 30 and 28.
In addition to exchanges of audiovisual information between the layers 30 and 28, a set of double-headed arrows 42 indicate bidirectional exchanging of video data between the H.221 layer 28 and an application layer 44 which may use received video data to generate a visible image. To provide the application layer 44 with video data, the H.221 layer 28 extracts such data from the audiovisual information that it receives from the B-channels 22. Correspondingly, the H.221 layer 28 embeds video data that it receives from the application layer 44 into the audiovisual information that it transmits to the B-channels 22. A set of double-headed arrows 46 indicate bidirectional exchanging of audio data between the H.221 layer 28 and the application layer 44 which may use the audio data to generate an audible sound. Similar to the processing of audiovisual information and video data by the H.221 layer 28, the layer 28 extracts audio data from the audiovisual information that it receives from the B-channels 22 and passes the audio data to the application layer 44. The H.221 layer 28 also embeds audio data received from the application layer 44 into the audiovisual information that it passes to the B-channels 22. Sets of double-headed arrows 48 and arrows 52 indicate bidirectional exchanging respectively of high-speed data and low-speed data between the H.221 layer 28 and the application layer 44. The H.221 layer 28 extracts the high-speed, and low-speed data from the audiovisual information that it receives from the B-channels 22 and transmits such data to the application layer 44, and it embeds the high-speed, and low-speed data that it receives from the application layer 44 into the audiovisual information that it transmits to the B-channels 22.
FIG. 2 illustrates the relationships that exists among the H.221 layer 28 and twenty-three (23) B-channels 22 and a D-channel 24 respectively carried by twenty-four (24) time-slots of an ISDN primary rate access 62. The B-channels 22 and the D-channel 24 of the ISDN primary rate access 62 implement the I.400 layer 30. In the illustration of FIG. 2, the CCITT Q.921 layer 34 and CCITT Q.931 layer 36 are superimposed over the time-slot of the ISDN primary rate access 62 which carries the D-channel 24. Similar to FIG. 1, the sets of double-headed arrows 38 extending between various of the B-channels 22 and the H.221 layer 28 indicate bidirectional exchanging of audiovisual information between the two layers 30 and 28. Two sets of six arrows 38 each in FIG. 2 represent two H0-connections 64. In the illustration of FIG. 2, one of the H0-connections 64 is made up of 6 B-channels 22 respectively carried in six immediately adjacent time-slots. FIG. 2 also illustrates six randomly assigned time-slots carrying the six B-channels 22 which make up the other H0-connection 64. As indicated by the sets of double-headed arrows 42, 46, 48 and 52 in FIG. 2 which are identical to the sets of double-headed arrows 42, 46, 48 and 52 in FIG. 1, other than for the rate of data transmission a H.221 layer 28 exchanging audiovisual information with one or more H0-connections 64 provided by an ISDN primary rate access 62 may present an identical interface to the application layer 44 for exchanging video, audio, high-speed, and low-speed data as the H.221 layer 28 illustrated in FIG. 1 that exchanges audiovisual information with three ISDN basic rate accesses 26.
FIG. 3 illustrates the relationship among the H.221 layer 28 and the B-channels 22 and the D-channel 24 of a H11-connection indicated by the general reference character 72. The H11-connection 72 is made up of an ISDN primary rate access 62 and an ISDN basic rate access 26. Similar to the illustrations of FIGS. 1 and 2, the ISDN primary rate access 62 combined with the ISDN basic rate access 26 depicted in FIG. 3 implement the I.400 layer 30 of the H11-connection 72. In accordance with the I.400 Recommendation for a H11-connection 72, the D-channel 24 of the ISDN basic rate access 26 carries control information used by the CCITT Q.921 layer 34 and the CCITT Q.931 layer 36. Thus in the H11-connection 72, all twenty-four (24) time-slots of the ISDN primary rate access 62 are used as B-channels 22 for exchanging audiovisual information with the H.221 layer 28 as indicated by the sets of double-headed arrows 38. As indicated by the sets of double-headed arrows 42, 46, 48 and 52 in FIG. 3 which are identical to the sets of double-headed arrows 42, 46, 48 and 52 in FIGS. 1 and 2, other than for the rate of data transmission a H.221 layer 28 exchanging audiovisual information with twenty-four B-channels 22 provided by a H11-connection 72 may present an identical interface to the application layer 44 for exchanging video, audio, high-speed, and low-speed data as the H.221 layer 28 respectively illustrated in FIG. 1 or in FIG. 2.
If two terminals exchange audiovisual information using a H11-connection 72, a H12-connection, or a single HO-connection 64, then ISDN's operation inherently preserves the inter B-channel phase relationship from the transmitting terminal's H.221 layer 28 to the receiving terminal's H.221 layer 28. For any other form of ISDN transmission permitted under CCITT's H.221 Recommendation, preservation of the phase relationship between B-channels 22 is not assured. Skew can occur between data received on different B-channels 22 even if the receiving terminal connects to a ISDN primary rate access 62 when the transmitting terminal connects to two or more B-channels 22 of ISDN basic rate accesses 26, or conversely. That is, if two or more H0-connections 64 or two or more B-channels 22 on an ISDN basic rate accesses 26 are used for transmitting or receiving audiovisual teleservices, data transmitted in-phase need not arrive in-phase at the receiving terminal. Such skew in received data may occur whenever audiovisual teleservices are transmitted over B-channels 22 established by placing different telephone calls. CCITT's ISDN Recommendation permits the inter B-channel skew between two H0-connections 64 or between two B-channels 22 of ISDN basic rate accesses 26 to be as great as 1,200 milliseconds. This skew between information carried in B-channels 22 of different H0-connections 64 or the different B-channels 22 of ISDN basic rate accesses 26 occurs because HO-connections 64 or B-channels 22 of ISDN basic rate accesses 26 may be transmitted between terminals over different paths.
While the CCITT H.221 Recommendation specifies a structure for digital audiovisual information transmitted over various different ISDN accesses or connections, it does not disclose nor does it suggest any digital circuit and/or computer hardware and/or computer software capable of effecting such a communication. Furthermore, the CCITT H.221 Recommendation provides no suggestion of an interface, i.e. data structures and protocols, for exchanging video, audio, high-speed, and low-speed data between the H.221 layer and an application program executed in a host computer.